Lead-free brass alloy for hot working

ABSTRACT

Provided is a lead-free brass alloy for hot working provided with good hot-working properties and mechanical characteristics. A lead-free brass alloy for hot working, comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, the zinc equivalent being in a range of 40.0 to 43.0, and the area ratio of the κ phase after hot working being 20% or less.

TECHNICAL FIELD

The present invention relates to a lead-free brass alloy for hotworking, having excellent resistance to dezincification and resistanceto erosion and corrosion, and having good hot-working properties andmechanical characteristics.

BACKGROUND ART

Bronze, brass, and other copper alloys have conventionally been used infaucet parts for water supply, water contact parts for general piping,and in various valves in order to make use the excellent materialcharacteristics of such alloys. These copper alloys require goodmachinability for working a product, and therefore lead has generallybeen included to thereby impart the required machinability. For example,JIS H5120 CAC406, CAC407, and other bronze alloys, and JIS H3250 C3604,C3771, and other brass alloys having excellent machinability contain 1to 6 wt % of lead.

However, lead evaporates in the alloy melting and casting process,elutes into drinking water when used as a water contact part, and hasother drawbacks. There is a deepening awareness that lead is a toxicelement that negatively affects the human body and environmentalsanitation, and the content of lead has been strictly restricted inincreasing fashion in recent years. Accordingly, there is a need todevelop a free-cutting copper alloy that does not contain lead.

In view of the background described above, in silzin bronze alloys, aCu—Zn—Si alloy in which free-cutting is achieved by adding siliconwithout the inclusion of lead has been proposed and used (see PatentDocuments 1 and 2). Additionally, there has been proposed a Cu—Zn—Si—Snalloy in which tin has been added in order to enhance the corrosionresistance of a Cu—Zn—Si alloy (see Patent Document 3). There have alsobeen proposed a Cu—Zn—Si—Bi alloy in which bismuth has been added inorder to further improve the machinability of a Cu—Zn—Si alloy (seePatent Document 4), and a Cu—Zn—Si—Sn—Bi alloy (see Patent Document 5)in which tin has been added to the Cu—Zn—Si—Bi alloy in order to improvecorrosion resistance. These alloys have excellent mechanicalcharacteristics and dezincification resistance, and excellentmachinability in the case that bismuth has been added, and alloys inwhich Bi has not be added are provided with excellent hot workability.In the case that bismuth is added to a Cu—Zn—Si alloy, there is anadditional advantage in that the alloy can be used as scrap to bedissolved into raw materials.

PRIOR ART DOCUMENTS Patent Documents

-   [Patent Document 1] Japanese Patent No. 3917304-   [Patent Document 2] Japanese Laid-open Patent Application No.    2001-64742-   [Patent Document 3] Japanese Laid-open Patent Application No.    2002-12927-   [Patent Document 4] Japanese Laid-open Patent Application No.    2009-7657-   [Patent Document 5] Japanese Patent Application No. 2010-84231

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

The alloys disclosed in the above-noted documents can be said to havethe main object of removing lead toxicity. Therefore, the most importantissue in terms of performance is to maintain free-cuttingcharacteristics without the inclusion of lead, and to a certain extentmachinability has been ensured.

However, in the case that the alloy does not contain bismuth,machinability is improved by the silicon compound, but the improvementmay be insufficient in some cases, and currently, a certain amount ofbismuth must be added in order to improve machinability. Also, it ispreferred the alloy contain bismuth from the viewpoint of use as scrap.

However, a bismuth-containing lead-free brass alloy can be hot worked inmold working in which there is little deformation, but in the case ofmolding work with a considerable amount of deformation, forge crackingor other defects readily occur unless the addition amount of bismuth andthe forging conditions are rigorously controlled. It is known that hotforging a brass alloy has different conditions in which cracking occursin a product depending on the working temperature. There are upper andlower limits to the working temperature at which working can beperformed without cracks forming, and heating and forging must becarried out in this temperature region (hereinafter referred to asworking temperature range). For example, the working temperature must beincreased for the alloy in Patent Document 5, which contains about 0.7wt % of bismuth, and since the working temperature range is very narrow,temperature management is difficult, and there is also a problem interms of the amount of energy used. Patent Document 3 describes an alloyin which it is effective to add silicon as the element for improving hotforging characteristics, but in the embodiments, there is no dataprovided in relation to the hot-working characteristics in the case thatbismuth has been added. The only evaluation is that the workingtemperature is on the single level of 750° C., and the workingtemperature range is unclear.

The inventors carried out studies and found that the working temperaturerange becomes very narrow when bismuth is included in a Cu—Zn—Si—Snalloy. Therefore, problems readily occur in operations because theforging conditions must be rigorously controlled in order to subjectthis alloy to molding work that involves a considerable amount ofdeformation. In other words, broadening the working temperature range isa first issue and is important in order to apply the excellent corrosionresistance and machinability of this alloy to a large number ofcomponents.

Also, tin is added in order to increase dezincification and erosion andcorrosion resistance, and elongation of a Cu—Zn—Si—Sn—Bi alloy isreadily reduced. The κ phase and γ phase of this alloy precipitate and,depending on the precipitation conditions, the mechanical properties arereadily degraded. Furthermore, these precipitation conditions arereadily affected by the heat history or the like during manufacture, andit is important to accurately ascertain and suitably control therelationship between the configuration of the structure and themechanical properties. In other words, a second issue is controlling themechanical properties, more particularly, the elongation of theCu—Zn—Si—Sn—Bi alloy.

The present invention was devised in order to solve the above-describedproblems, it being an object thereof to provide a lead-free brass alloyfor hot working provided with good hot-working properties and mechanicalcharacteristics.

Means for Solving the Problems

The main points of the present invention are described below.

A first aspect of the present invention relates to a lead-free brassalloy for hot working, characterized in comprising: 28.0 to 35.0 wt %zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt %bismuth, 0.10 wt % or less lead, and the remainder being copper andunavoidable impurities, the zinc equivalent being in a range of 40.0 to43.0, and the area ratio of the κ phase after hot working being 20% orless.

The present invention also relates to the lead-free brass alloy for hotworking according to the first aspect, characterized in that elongationis 10% or more.

Effects of the Invention

The present invention is configured in the manner described above, andis therefore a lead-free brass alloy for hot working, provided with goodhot-working properties and mechanical characteristics. In other words,adding 28.0 to 35.0 wt % zinc makes it possible to obtain goodhot-working properties. In similar fashion to zinc, silicon is anessential element for obtaining good hot-working properties and theaddition of 0.5 to 2.0 wt % is effective. Tin contributes to improvementin dezincification decay and resistance to erosion and corrosion decay.Bismuth is added in order to improve machinability. The zinc equivalentis determined by the balance among zinc, silicon, and other elements,and is a parameter for maintained a balance between hot-workingproperties and mechanical characteristics in particular. A zincequivalent of 40.0 to 43.0 simultaneously satisfies the twocharacteristics. Also, the area ratio of the κ phase after hot workingis 20% or less, whereby good mechanical characteristics are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a description of the zinc equivalent;

FIG. 2 is a chart showing the chemical components of samples used in thehot-working test;

FIG. 3 is a descriptive view showing the shape of the test piece in thehot-working test;

FIG. 4 is a chart showing the forging test results;

FIG. 5 is a graph showing the relationship between the Si additionamount and the working temperature range;

FIG. 6 is a graph showing the relationship between the Zn equivalent andthe working temperature range;

FIG. 7 is a chart showing the chemical components of samples used in thetensile test;

FIG. 8 is a chart showing the test results of the tensile test;

FIG. 9 is a graph showing the relationship between the Si additionamount and the mechanical characteristics in a low Zn equivalent;

FIG. 10 is a graph showing the relationship between the Si additionamount and the mechanical characteristics in a high Zn equivalent;

FIG. 11 is a chart showing the chemical components of samples in whichthe relationship between the Si addition amount, the area ratio of the κphase, and the elongation has been studied;

FIG. 12 is a chart showing the relationship between the Si additionamount, the area ratio of the κ phase, and the elongation;

FIG. 13 is a graph showing the relationship between the Si additionamount and the area ratio of the κ phase;

FIG. 14 is a graph showing the relationship between the area ratio ofthe κ phase and the elongation;

FIG. 15 is a chart showing the chemical components of samples used inthe erosion and corrosion test, and the dezincification decay test;

FIG. 16 is a descriptive view showing the shape of the test piece in theerosion and corrosion test;

FIG. 17 is a chart showing the test conditions;

FIG. 18 is a chart showing the test results;

FIG. 19 is a chart showing the test results of the dezincification decaytest;

FIG. 20 is a chart showing the chemical components of samples used inthe machinability test;

FIG. 21 is a chart showing the test conditions;

FIG. 22 is a chart showing the test results; and

FIG. 23 is a photograph showing an example of the photographedmicrostructure.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention are briefly describedbelow while indicating the effects of the present invention.

In order to obtain good resistance to dezincification and resistance toerosion and corrosion, and to provide good hot-working properties andmechanical characteristics, the present invention provides a lead-freebrass alloy for hot working comprising: 28.0 to 35.0 wt % zinc, 0.5 to2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt% or less lead, and the remainder being copper and unavoidableimpurities, wherein the zinc equivalent is in a range of 40.0 to 43.0.

The component composition as described above in the present invention,the reasons for specifying the mechanical characteristics, and theeffects of the present invention will be briefly described below.

Zinc (Zn)

Zinc dissolves in the matrix of a Cu—Zn—Si copper alloy, and has theeffect of increasing mechanical strength. Zinc also reduces the meltingpoint of the alloy, increases the fluidity of the molten alloy, andenhances casting characteristics. Zinc also has the effect of improvinghot working, and in order to obtain these effects, zinc must be added inthe amount of 28.0 wt % or more due to the relationship between thelater-described silicon addition amount and the zinc equivalent.

However, when the amount of zinc exceeds 35.0 wt %, the hot-workingproperties are conversely degraded due to the relationship between thelater-described silicon addition amount and the zinc equivalent. Also,the mechanical characteristics of the alloy are liable to be degradeddue to precipitation of a hard phase that is greater than necessary. Dueto such reasons, the zinc content is set to 28.0 to 35.0 wt %.

Silicon (Si)

Silicon works as a deoxidizer during dissolution, enhances the fluidityof the molten alloy, and improves casting characteristics. A portiondissolves in the matrix and improves mechanical strength, and a portionworks with zinc to cause the emergence of a hard phase that functions asa chip breaker during cutting work and improve machinability.

Furthermore, as a result of thoroughgoing research, the presentinventors discovered the following, which dramatically improves theworking temperature range (a value obtained by subtracting the lowerlimit from the upper limit of the working temperature in which hotforging can be carried out without the occurrence of cracking) of aCu—Zn—Sn—Si alloy in the case that bismuth is included.

In the heating stage during hot working, bismuth has a property ofreadily aggregating at the grain boundary, and this is thought to be thecause of inhibiting hot-working properties. However, the addition of asuitable amount of silicon prevents bismuth aggregation and is effectivein preventing forging cracks. In order to obtain these effects, siliconmust be added in the amount of 0.5 wt % or more. When the contentexceeds 2.0 wt %, hot-working properties are degraded even when the zincequivalent has been kept at an optimal level, and the mechanicalcharacteristics of the alloy are liable to be degraded due to theemergence of a hard phase that is greater than necessary. Due to suchreasons, the silicon content is set to 0.5 to 2.0 wt %.

Tin (Sn)

Tin is effective for enhancing dezincification resistance and resistanceto erosion and corrosion. Tin is particularly effective in improvingerosion and corrosion properties, and in order to obtain these effects,tin must be added in the amount of 0.5 wt % or more. On the other hand,when the content exceeds 1.5 wt %, mechanical characteristics are liableto be degraded. Due to such reasons, the tin content is set to 0.5 to1.5 wt %.

Bismuth (Bi)

A bismuth content less than 0.5 wt % can be considered to have littleeffect for improving machinability, but machinability is improved inaccordance with the addition amount by adding 0.5 wt % or more. However,the addition of a large amount is not preferred in that it causesdegradation in hot-working properties. A large amount not only causesdegradation in hot-working properties, but also causes degradation inthe mechanical characteristics, hence the upper limit is set to 1.5 wt%.

Lead (Pb)

The lead content is set to 0.10 wt % or less, and it is thereby possibleto essentially avoid evaporation in the dissolution and castingprocesses of the alloy, as well as lead poisoning in the human bodyand/or environmental hygiene due to elution into drinking water or thelike when the alloy is used as a water contact component. Due to suchreasons, the lead content is limited to 0.10 wt % or less.

Copper (Cu)

Copper is an element that reduces sensitivity to dezincification decayand improves corrosion resistance and mechanical characteristics, but inthe alloy of the present invention, the copper content is determined asthe remainder due to the balance between the zinc content and siliconcontent. The effective content is 59.0 to 71.0 wt %.

Zinc Equivalent

The zinc equivalent is an important parameter for maintaining a broadworking temperature range in the alloy of the present invention. Asdescribed above, a suitable addition of silicon makes it possible tomaintain a broad working temperature range, but control is insufficientusing silicon alone, and using the zinc equivalent computed by thebalance between silicon, zinc, and the like to achieve limited controlmakes it possible to more reliably maintain a broad working temperaturerange. The present inventors carried out research and found that settingthe zinc equivalent in the alloy of the present invention to 40.0 ormore provides a working temperature range that is broad enough tosatisfy industrial requirements. However, a zinc equivalent exceeding43.0 is liable to lead to degradation in the mechanical characteristics.In view of this background, the zinc equivalent is set to 40.0 to 43.0.

The zinc equivalent is obtained using the Guillet formula (zincequivalent=100×(B+Σtq)/(A+B+Σtq)), and the zinc equivalent of Bi iscalculated using a factor of 1 (see FIG. 1).

κ Phase Quantitative Ratio or Heat Treatment

The addition of the elements described above and the use of hot workingmakes it possible to demonstrate the excellent function of the alloy ofthe present invention, but depending on the cooling speed and/or theprocessing rate during hot working, elongation may be slightlyinsufficient. In order to improve the elongation of the alloy of thepresent invention, the metal structure must be controlled, and settingthe area ratio of the κ phase in the alloy of the present invention to20% or less makes it possible to ensure elongation. Therefore, the arearatio of the κ phase is set to 20% or less. The method for controllingthe structure is not particularly limited, and may be controlled using ahot-working method, heat treatment, or the like.

Embodiments

Specific embodiments of the present invention will be described belowwith reference to the drawings.

The alloy according to the present invention (the alloy of the presentinvention) and a comparative alloy were used as samples and tested inthe manner described below.

1) Hot-Working Test

The chemical components of the samples used in the hot-working test areshown in FIG. 2. A molten alloy melted in a Siliconit furnace for testdissolution and prepared with the chemical components shown in FIG. 2was cast in a mold having an outside diameter of 88 mm and a length of120 mm, and then machined to an outside diameter of 78 mm and a lengthof 90 mm. Machined billets were extruded to a diameter of 22 mm, and theresulting rods were worked into a test piece shape such as that shown inFIG. 3. The working temperature was varied and these test pieces wereforged with a processing rate of 80%. As used herein, the processingrate is calculated using the following formula.

Processing rate=100×(sample height prior to forging−sample height afterforging)/sample height prior to forging

The test pieces (samples) after forging were observed macroscopically,the lower limit was subtracted from the upper limit of the workingtemperature at which forging can be carried out without the occurrenceof cracking, and this was used to define the working temperature rangeand make evaluations. The heating time in all tests was 20 minutes. Theworking temperature range of each sample is shown in FIGS. 4 to 6.

(a) Effectiveness of the Silicon Addition

The effectiveness of adding silicon to the alloy of the presentinvention is shown in FIG. 5. In the case that silicon is not added, theworking temperature range is narrow, but it is apparent that the workingtemperature range increases in accompaniment with the addition ofsilicon. The effect of these additions is to produce a satisfactoryworking temperature range with the addition of 0.5 wt % or more.However, when the addition amount exceeds 2.0 wt %, the workingtemperature range tends to be conversely reduced, and it was found thatan effective silicon content is 0.5 to 2.0 wt %.

(b) Effectiveness of the Zinc Equivalent

Next, the effectiveness of the zinc equivalent is shown in FIG. 6. Itwas found that the zinc equivalent must be 40.0 to 43.0 in order toadequately maintain the working temperature range in the alloy of thepresent invention, and it was confirmed that the zinc equivalent must becontrolled, as appropriate, in accordance with the effect of increasingthe working temperature range by the silicon addition described above.

2) Tensile Test of the Hot-Working Material

The chemical components of the sample materials used in the tensile testare shown in FIG. 7. A molten alloy was cast in a mold having a diameterof 45 mm and a length of 100 mm, and was then machined into billetshaving a diameter of 40 mm and a length of 75 mm. The billets weresubsequently heated to 650 to 750° C. and extruded to a diameter of 10mm, then machined into test pieces in accordance with JIS Z2201 14A, andsubjected to a tensile test using a universal testing machine. Theresults are shown in FIGS. 8 to 10.

When the effect of the silicon addition amount is considered, there is anoted tendency for the elongation to be reduced in accordance with thesilicon addition amount, and this is particularly dramatically in thecase that the zinc equivalent is high. It is apparent that the tensilestrength tends to be temporarily reduced when the silicon content isnear 1.0 wt % with the zinc equivalent near 40.6, and when the siliconcontent is near 2.0 wt % with the zinc equivalent near 42.5, butthereafter the tensile strength increases.

3) Metal Structure and Mechanical Characteristics

The alloy of the present invention has excellent hot-working propertiesas described above, and it is important to suitably control the Siaddition amount and the zinc equivalent. However, elongation tends to bereadily reduced when the zinc equivalent is high, and controlling thestructure also becomes an issue.

The alloy of the present invention mainly has κ-phase and α-phaseconstituent structures, and between these two, the structure wasobserved with focus on the effect that the quantitative ratio of the κphase has on the mechanical characteristics. Five locations werephotographed using an optical microscope to obtain images at 500×magnification using the samples used in the tensile test describedabove. The quantitative ratio of the κ phase was measured using imageprocessing software (an example of the photographs taken is shown inFIG. 23). These results are shown in FIGS. 11 to 14. The inventors foundthe following facts from these structural observations. The elongationof the alloy of the present invention was found to have a very strongcorrelation with the area ratio of the κ phase, and when elongation isto be increased, the area ratio of the κ phase must be kept low.

The relationship between the area ratio of the κ phase and the siliconaddition amount increases in accordance with the silicon addition amount(see FIG. 13). In terms of the relationship between the area ratio ofthe κ phase and the silicon addition amount, elongation is 10% or morewhen the area ratio of the κ phase is 20% or less (see FIG. 14).Therefore, the area ratio of the κ phase in the alloy of the presentinvention must be 20% or less.

4) Corrosion Decay Test

(a) Erosion and Corrosion Test

The chemical components of the sample materials used in the erosion andcorrosion test are shown in FIG. 15. A molten alloy melted in aSiliconit furnace for test dissolution and prepared with the chemicalcomponents shown in FIG. 15 was cast in a mold having a diameter of 40mm and a length of 100 mm, and was then worked into a test piece shapesuch as that shown in FIG. 16. Testing was carried out with the testconditions of FIG. 17 using these test pieces. The test results areshown in FIG. 18. It was found from these results that the alloy of thepresent invention was slightly inferior to CAC406, but was considerablybetter than free-cutting brass.

(b) Dezincification Decay Test

The same samples as those used in the erosion and corrosion test wereused. The test was carried out in accordance with ISO 6509. The testresults are shown in FIG. 19. Good results were obtained with the alloyof the present invention in that the maximum decay depth was 100 μm orless for all samples.

5) Machinability Test

The chemical components of the sample materials used in the erosion andcorrosion test are shown in FIG. 20. A molten alloy melted in aSiliconit furnace for test dissolution and prepared with the chemicalcomponents shown in FIG. 20 was cast in a JIS H5120 E mold, the outsidediameter of the test pieces was worked using the cutting conditionsshown in FIG. 21, and the cutting resistance of the test pieces wasmeasured. The test results are shown in FIG. 22. In comparison withlead-containing bronze and lead-containing brass, the alloy of thepresent invention has higher resistance, but is on the same level asthat of lead-free bronze.

In view of the above, it was confirmed that the lead-free brass alloyfor hot working has good hot-working properties and mechanicalcharacteristics, the lead-free brass alloy for hot working, comprising:28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin,0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder beingcopper and unavoidable impurities, wherein the zinc equivalent is in arange of 40.0 to 43.0.

1. A lead-free brass alloy for hot working, characterized in comprising:28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin,0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder beingcopper and unavoidable impurities, the zinc equivalent being in a rangeof 40.0 to 43.0, and the area ratio of the κ phase after hot workingbeing 20% or less.
 2. The lead-free brass alloy for hot workingaccording to claim 1, characterized in that elongation is 10% or more.